A holographic imaging device for ultrafast light source

Abstract

The present invention discloses a holographic imaging and device applied to ultrafast light sources, belonging to the field of microscopic imaging technology. The method comprises: when using a broadband light source, respectively collecting background spot diffraction data and sample diffraction data, dividing the sample diffraction data by the background spot diffraction data to obtain a normalized hologram; applying amplitude limitation and phase limitation to the sample function in the near-field plane to obtain a reconstructed image with no twin images at all. The present invention applies holographic imaging technology to ultrafast imaging, and only needs to input a pair of diffraction patterns to realize large-field super-resolution holographic imaging of broadband light sources. Compared with the stacking algorithm, the time spent on reconstruction is greatly reduced, and the time spent on collecting data is also much less than that of the stacking algorithm; the sampling density of the spectrum can be improved, and the loss of resolution of the reconstructed image can be reduced.

Description

Technical Field

[0001] The present invention belongs to the field of microscopic imaging technology, and more specifically, relates to a holographic imaging device applied to an ultrafast light source. Background Art

[0002] The human eye's visual persistence time is generally 0.1-0.4 seconds. When the change process is shorter than 0.1 second, it is impossible to see the change process clearly due to the limitation of the eye's time resolution. For example, to see clearly whether the four feet of a high-speed running horse leave the ground at the same time, millisecond time resolution is required, which requires high-speed photography technology. High-speed photography in movies uses a high-speed camera to record the fleeting rapid change process and project it in slow motion, so as to show the instantaneous movement that is invisible to the naked eye. For some physical processes with shorter time scales (such as femtoseconds or even attoseconds), in order to observe such ultra-high-speed phenomena or ultra-fast processes, faster photography technology is needed, which depends on the revolutionary progress of rapid detection methods and technologies. How to achieve a complete and accurate record of a transient process through a new ultrafast imaging method has always been an important scientific issue that has received widespread attention in the field of ultrafast process research.

[0003] Technological advances in attosecond science promise imaging methods that combine attosecond temporal resolution with nanometer spatial resolution. Tabletop high-order harmonic generation (HHG) sources can generate isolated attosecond pulses that are naturally synchronized with the driving laser field, making them ideal for pump-probe studies with extremely high temporal resolution. So far, many partially coherent algorithms based on coherent diffraction imaging (CDI) have been proposed. For example, the mixed-state algorithm proposed by Pierre Thibault et al. is used to quantitatively image targets illuminated by ultrafast light sources. However, the ms algorithm almost fails when the spectral bandwidth exceeds 2%. The ployCDI algorithm proposed by Brian Abbey et al. can be applied to both continuous and discrete spectra. The maximum tolerance for bandwidth is 11%, and the exposure time for broadband data acquisition is reduced by 60 times. Recently, broadband CDI was proposed by Huijts et al. They monochromatized the broadband diffraction pattern obtained by the detector. The monochromatized diffraction data are suitable for any CDI algorithm and can achieve almost the same resolution as a monochromatic source at a bandwidth of 11%. Both ployCDI and broadband CDI use a single-frame diffraction image for reconstruction, which results in their very small field of view and are particularly sensitive to noise. In contrast, coaxial holography can achieve single-frame imaging with high robustness and a large field of view. And because there is no central bright spot in the hologram, the requirements for the dynamic range of the detector are greatly reduced.

[0004] When coaxial holography is applied to ultrafast imaging, the twin image problem in the hologram must be solved first: in traditional holographic imaging, the twin image will be superimposed on the reconstructed object, which greatly limits the application of coaxial holography. In addition, coaxial holographic imaging technology is developed based on the good monochromaticity of the illumination light, which is essentially contradictory to the inherent wide spectrum of ultrafast light sources. Therefore, it is necessary to expand coaxial holographic technology to broadband light sources. The imaging resolution of coaxial holography is also limited by the pixel size of the detector. Summary of the invention

[0005] In view of the above defects or improvement needs of the prior art, the present invention provides a holographic imaging method and device applied to an ultrafast light source, which aims to reduce the acquisition and reconstruction time of experimental data and obtain a resolution almost consistent with that using a monochromatic light source.

[0006] To achieve the above object, according to one aspect of the present invention, a holographic imaging method applied to an ultrafast light source is provided, comprising:

[0007] S1. Using an ultrafast light source to illuminate the pinhole, so that the illumination spot formed by the diffraction of the ultrafast light source is incident on the sample plane, and the background light spot hologram and the sample diffraction hologram are collected respectively to obtain a normalized hologram;

[0008] S2. The result obtained by inversely transferring the normalized hologram back to the sample plane is used as the initial sample function;

[0009] S3. applying an amplitude limit to the current sample function on the sample plane so that the amplitude of the sample function is not greater than a set constant; and applying a phase limit so that the phase of the transmitted light part of the sample function is a constant phase;

[0010] S4. transferring the sample function to the detector plane and calculating the light field intensity value of the detector plane;

[0011] S5. Determine whether the error between the light field intensity value on the detector plane and the normalized hologram is less than the set value; if so, output the sample function; if not, perform amplitude replacement on the diffracted light field corresponding to the central wavelength on the detector plane, and transfer it back to the sample plane, and return to execute S3.

[0012] Furthermore, a noise pattern is introduced into the light field intensity value for compensation.

[0013] Furthermore, the noise pattern is updated in each iteration:

[0014]

[0015] I(R) is the calculated light field intensity value of the detector plane, I m (R) is the normalized hologram; ns j(R) represents the additional noise pattern function after the jth iteration.

[0016] Furthermore, the method before step S5 further includes: applying a Lucy-Richardson deconvolution algorithm to process the single-frame hologram.

[0017] Furthermore, the distance between the sample and the detector is obtained in the following way: a distance interval is scanned, and the image quality of the obtained traditional holographic result is judged by using the sobey function, and the distance corresponding to the result with the largest sobey value is taken as the distance between the sample and the detector.

[0018] Furthermore, the distance Z0 between the detector and the sample and the distance Z1 between the sample and the pinhole satisfy:

[0019]

[0020] The present invention also provides a holographic imaging device applied to an ultrafast light source, comprising:

[0021] An ultrafast light source is used to emit an ultrafast light source to illuminate the pinhole, so that the illumination spot formed by the diffraction of the ultrafast light source is incident on the sample plane;

[0022] A processing module, used for collecting the background spot hologram and the sample diffraction hologram respectively to obtain a normalized hologram;

[0023] An iterative reconstruction module is used for inversely transferring the normalized hologram back to the sample plane to obtain a result as an initial sample function;

[0024] An amplitude limit is imposed on the current sample function on the sample plane so that the amplitude of the sample function is not greater than a set constant; at the same time, a phase limit is imposed so that the phase of the transmitted light part of the sample function is a constant phase;

[0025] The sample function is transferred to the detector plane, and the light field intensity value of the detector plane is calculated;

[0026] Determine whether the error between the light field intensity value on the detector plane and the normalized hologram is less than the set value; if so, output the sample function; if not, perform amplitude replacement on the diffracted light field corresponding to the central wavelength on the detector plane, and transfer it back to the sample plane, and return to execute the amplitude limitation and phase limitation process.

[0027] In general, compared with the prior art, the above technical solution conceived by the present invention can achieve the following beneficial effects.

[0028] (1) The present invention applies holographic imaging technology to ultrafast imaging. Only one diffraction pattern needs to be input to achieve large-field super-resolution holographic imaging of a broadband light source. Compared with the stacking algorithm, the time spent on reconstruction is greatly reduced, and the time spent on data collection is also much less than that of the stacking algorithm. The sampling density of the spectrum can be improved, reducing the loss of resolution of the reconstructed image.

[0029] (2) The present invention adds an additional noise mode to compensate for the influence of various uncertain factors, such as laser jitter, fluctuation of photon count, error of detector readout circuit, etc., to avoid the semi-convergence or even failure of the phase iteration process.

[0030] (3) The present invention introduces the Lucy-Richardson deconvolution algorithm to double the spatial resolution of the reconstructed image.

[0031] (4) The present invention sets the distance between the detector and the sample to be much larger than the distance between the sample and the pinhole, so that the light field incident on the sample plane can be regarded as a plane wave. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 A flow chart of a holographic imaging method using an ultrafast light source provided by an embodiment of the present invention;

[0033] Figure 2 A schematic diagram of holographic imaging using an ultrafast light source provided by an embodiment of the present invention;

[0034] Figure 3 A flow chart of a phase iteration algorithm for holographic imaging using an ultrafast light source provided by an embodiment of the present invention;

[0035] Figure 4 (a)-(d) are reconstruction results of a broadband light source experiment at 5% bandwidth provided by an embodiment of the present invention using the method of the present invention;

[0036] Figure 5 (a)-(d) are super-resolution reconstruction results of a broadband light source experiment and a monochromatic light source experiment at 5% bandwidth provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0038] refer to Figure 1The present invention provides a holographic imaging method applied to an ultrafast light source, comprising:

[0039] S1. Using an ultrafast light source to illuminate the pinhole, so that the illumination spot formed by the diffraction of the ultrafast light source is incident on the sample plane, and the background light spot hologram and the sample diffraction hologram are collected respectively to obtain a normalized hologram;

[0040] like Figure 2 As shown in the figure, a collimated broadband light source illuminates the precision pinhole. The light source passing through the precision pinhole forms a large illumination spot in the far field and is incident on the sample plane. The detector placed behind the sample collects the background spot diffraction pattern and the sample diffraction pattern under the same exposure time.

[0041] Use the background spot diffraction data and sample diffraction data for normalization:

[0042]

[0043] Where R represents the far field coordinate, I s (R) represents the sample diffraction data, I b (R) represents the background spot diffraction data, I m (R) is the normalized diffraction data.

[0044] Preferably, the light spot formed in the far field after the light source passes through the precision pinhole should have its central bright spot covering the chip of the detector as much as possible.

[0045] Preferably, in order to make the light field incident on the sample plane approximate to a plane wave, the distance Z0 between the sample and the pinhole should be much larger than the distance Z1 between the sample and the detector.

[0046]

[0047] S2. The result obtained by inversely transferring the normalized hologram back to the sample plane is used as the initial sample function;

[0048] S3. applying an amplitude limit to the current sample function on the sample plane so that the amplitude of the sample function is not greater than a set constant; and applying a phase limit so that the phase of the transmitted light part of the sample function is a constant phase;

[0049] At each iteration, amplitude and phase constraints are imposed on the sample function in the near field:

[0050] |ψ(r)|≤C

[0051] φ(|ψ(r)|=1)=φ constant

[0052] Where r represents the near-field coordinate, ψ(r) represents the sample function, φ(r) represents the phase distribution of the sample function, and C is a constant less than 1. The physical meaning of amplitude limitation is energy conservation, that is, the amplitude cannot increase after the light field is incident on the sample. The physical meaning of phase limitation is that the amplitude and phase of the part of the light field that directly transmits through the sample are considered to be basically consistent, while the present invention only cares about the relative phase of the sample function. Therefore, in each iteration, the phase of the transmitted light part of the sample function is set to a constant phase. The imposed phase limitation is very beneficial to image reconstruction and can completely eliminate the twin images in traditional holography.

[0053] In each experiment, the maximum amplitude value set needs to be adjusted for different samples, that is, the constant C in the above formula. The sample substrate used in the embodiment of the present invention is 1 mm soda-lime glass, whose transmittance to visible light is about 0.95. Therefore, setting the maximum amplitude value of the sample's transmission function to 0.95 instead of 1 will be more conducive to reconstruction.

[0054] S4. transferring the sample function to the detector plane and calculating the light field intensity value of the detector plane;

[0055] The distance between the sample and the camera is small enough, and the sample function after the restriction is propagated to the far-field plane, that is, the detector plane, using the angular spectrum transfer function:

[0056]

[0057]

[0058] and denote Fourier transform and inverse Fourier transform, respectively; Ψ(R) is the light field function on the detector plane; H is the angular spectrum transfer function; z is the distance between the sample and the detector; k is the wave vector; λ is the wavelength; and f x ,f y is the spatial frequency.

[0059] Preferably, the outgoing light field corresponding to the center wavelength with the largest weight in the spectrum is selected for propagation, and the present invention believes that the response of the sample function to different wavelengths within the spectrum range is almost the same. In order to replace the amplitude of the far-field light field function, the light field intensity value of the far field needs to be calculated in each iteration:

[0060]

[0061]

[0062] Among them, Φ λ (R) represents the calculated far-field light function of different wavelengths, ξ iis the weight coefficient of different wavelengths, I is the calculated far-field light intensity value, H λ , They represent the angular spectrum transfer function of different wavelengths and the sample function corresponding to the central wavelength. In the above formula, since the integration time of the camera is much longer than the coherence time of the light fields of different wavelengths, the present invention assumes that the detector records the incoherent superposition of the diffracted light fields corresponding to different wavelengths.

[0063] Preferably, since only a single frame of diffraction pattern is input for iterative reconstruction, the broadband reconstruction process is sensitive to noise, which may cause iterative semi-convergence or convergence failure in actual situations. The present invention introduces a noise mode to compensate:

[0064]

[0065] ns(R) represents the additive noise mode

[0066] And update the noise pattern in each iteration:

[0067]

[0068] I(R) is the far-field light intensity value obtained, I m (R) is the normalized diffraction data, ns j (R) represents the noise pattern after the jth iteration.

[0069] Whether using a monochromatic or broadband source, the spatial resolution of the reconstructed image of the hologram will be limited by the pixel size on the sensor chip. Since the angular spectrum transfer function is used, the resolution of the reconstructed image is theoretically only related to the pixel size of the detector. Here, by applying the Lucy-Richardson deconvolution algorithm on a single normalized hologram, the spatial resolution of the reconstructed image can be doubled:

[0070]

[0071] U and D represent upsampling and downsampling, respectively. h is the point spread function of the detector. Usually, the response distribution within each pixel is approximated by a two-dimensional Gaussian function. is the convolution symbol

[0072] S5. Determine whether the error between the light field intensity value on the detector plane and the normalized hologram is less than the set value; if so, output the sample function; if not, perform amplitude replacement on the diffracted light field corresponding to the central wavelength on the detector plane, and inversely transfer it back to the sample plane, and return to execute S3:

[0073]

[0074]

[0075] represents the angular spectrum transfer function corresponding to the central wavelength, Represents the light field function of the detector plane corresponding to the updated central wavelength.

[0076] The embodiment of the present invention is based on the use of a broadband light source as the incident light for holographic imaging, and combines a super-resolution method to double the spatial resolution of the reconstructed image. The experimental optical path of the present invention is simple and convenient to set up, and the experimental data acquisition operation is simple and fast. Since only one normalized diffraction pattern is input for reconstruction, a high-fidelity reconstructed image with a large field of view is obtained while greatly reducing the time required for calculation.

[0077] The method of the present invention is applicable to both monochromatic light sources and broadband light sources. Compared with existing methods, the method has simple equipment, fast reconstruction speed and high precision, and has great application prospects in the field of ultrafast microscopic imaging.

[0078] The embodiment of the present invention also provides a holographic imaging device using an ultrafast light source, comprising: an ultrafast light source, a precision pinhole, a processing module and an iterative reconstruction module;

[0079] An ultrafast light source illuminates a precision pinhole, causing the broadband light source to diffract to form an illumination spot in the far field and irradiate the sample.

[0080] The processing module is used to collect the diffraction image of the sample and the background light spot diffraction image when the sample moves out of the light path under the same exposure time; the background light spot diffraction image is subtracted from the diffraction image of the sample to obtain a normalized diffraction image;

[0081] An iterative reconstruction module is used for inversely transferring the normalized hologram back to the sample plane to obtain a result as an initial sample function;

[0082] An amplitude limit is imposed on the current sample function on the sample plane so that the amplitude of the sample function is not greater than a set constant; at the same time, a phase limit is imposed so that the phase of the transmitted light part of the sample function is a constant phase;

[0083] The sample function is transferred to the detector plane, and the light field intensity value of the detector plane is calculated;

[0084] Determine whether the error between the light field intensity value on the detector plane and the normalized hologram is less than the set value; if so, output the sample function; if not, perform amplitude replacement on the diffracted light field corresponding to the central wavelength on the detector plane, and transfer it back to the sample plane, and return to execute the amplitude limitation and phase limitation process.

[0085] The method and device provided by the present invention are further described below with a specific example.

[0086] A broadband light source with a central wavelength of 760nm and a bandwidth of 38nm was used as the incident light, and the diameter of the precision pinhole was 50μm. The sample used was a USAF1951 resolution test target, and the distance between the sample and the precision pinhole was about 20mm. A spectrometer was used to collect spectral information. The pixel size of the detector was 6.5μm, and the number of pixels was 1024. The propagation was carried out with the central wavelength as the reference. We input the normalized diffraction data into Figure 3 In the broadband holographic algorithm shown. Figure 4 (a) is the spectrum information of the light source, bandwidth The central wavelength is 760nm. Figure 4 (b) is a broadband normalized hologram. Figure 4 (c) is the reconstructed image obtained, the twin image is still well eliminated with almost no loss of spatial resolution. The field of view of the reconstructed image is 6656μm*6656μm. Figure 4 (d) is Figure 4 (c) The enlarged result at the dotted line.

[0087] The fourth and fifth groups of results of the samples were directly reconstructed using the monochromatic holograms collected by the detector. Figure 5 As shown in (a), the resolution is equal to the detector pixel size Δx LR = 6.5 μm. After combining the pixel super-resolution method, Figure 5 As shown in (b), the resolution of the reconstructed image is significantly improved, about Δx SR =3μm. The fourth and fifth groups of results of the sample were directly reconstructed using the broadband holograms collected by the detector. Figure 5 As shown in (c), after combining the pixel super-resolution method, the reconstruction result is as follows Figure 5 As shown in (d).

[0088] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A holographic imaging method applied to an ultrafast light source, characterized in that: include: S1. Using an ultrafast light source to illuminate the pinhole, so that the illumination spot formed by the diffraction of the ultrafast light source is incident on the sample plane, and the background light spot hologram and the sample diffraction hologram are collected respectively to obtain a normalized hologram; S2. The result obtained by inversely transferring the normalized hologram back to the sample plane is used as the initial sample function; S3. applying an amplitude limit to the current sample function on the sample plane so that the amplitude of the sample function is not greater than a set constant; and applying a phase limit so that the phase of the transmitted light part of the sample function is a constant phase; S4. transferring the sample function to the detector plane and calculating the light field intensity value of the detector plane; S5. Determine whether the error between the light field intensity value on the detector plane and the normalized hologram is less than the set value; if so, output the sample function; if not, perform amplitude replacement on the diffracted light field corresponding to the central wavelength on the detector plane, and transfer it back to the sample plane, and return to execute S3.

2. The holographic imaging method applied to an ultrafast light source according to claim 1, characterized in that: A noise pattern is introduced to the light field intensity values ​​to compensate.

3. The holographic imaging method applied to an ultrafast light source according to claim 2, characterized in that: Update the noise pattern in each iteration: I(R) is the calculated light field intensity value of the detector plane, I m (R) is the normalized hologram; ns j (R) represents the additional noise pattern function after the jth iteration.

4. The holographic imaging method applied to an ultrafast light source according to claim 2, characterized in that: The method before step S5 also includes: applying the Lucy-Richardson deconvolution algorithm to process the single-frame hologram.

5. The holographic imaging method applied to an ultrafast light source according to claim 2, characterized in that: The distance between the sample and the detector is obtained in the following way: scan a distance interval, use the sobey function to judge the image quality of the traditional holographic result, and take the distance corresponding to the result with the largest sobey value as the distance between the sample and the detector.

6. The holographic imaging method applied to an ultrafast light source according to claim 5, characterized in that: The distance Z0 between the detector and the sample and the distance Z1 between the sample and the pinhole satisfy:

7. A holographic imaging device applied to an ultrafast light source, characterized in that: include: An ultrafast light source is used to emit an ultrafast light source to illuminate the pinhole, so that the illumination spot formed by the diffraction of the ultrafast light source is incident on the sample plane; A processing module, used for collecting the background spot hologram and the sample diffraction hologram respectively to obtain a normalized hologram; An iterative reconstruction module is used to obtain a result of inversely transferring the normalized hologram back to the sample plane as an initial sample function; An amplitude limit is imposed on the current sample function on the sample plane so that the amplitude of the sample function is not greater than a set constant; at the same time, a phase limit is imposed so that the phase of the transmitted light part of the sample function is a constant phase; The sample function is transferred to the detector plane, and the light field intensity value of the detector plane is calculated; Determine whether the error between the light field intensity value on the detector plane and the normalized hologram is less than the set value; if so, output the sample function; if not, perform amplitude replacement on the diffracted light field corresponding to the central wavelength on the detector plane, and transfer it back to the sample plane, and return to execute the amplitude limitation and phase limitation process.

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